1. Field of the Invention
The present invention relates to a resist patterning process, comprising forming a positive pattern by exposure and development, making the positive pattern soluble in an alkaline liquid, coating a reverse film on it, and then reversing the positive pattern to a negative pattern by an alkaline etching.
2. Description of the Related Art
In recent years, as LSI progresses toward a higher integration and a further acceleration in speed, a finer pattern rule is required. In the light-exposure used as a general technology nowadays, the resolution inherent to the wavelength of a light source is approaching to its limit. In 1980s, a g-line (436 nanometers) or an i-line (365 nanometers) of a mercury lamp was used in a resist patterning process as an exposure light. As a means for further miniaturization, shifting to a shorter wavelength of an exposing light was assumed to be effective. As a result, in a weight production process after a DRAM (Dynamic Random Access Memoir) with a 64-Mega bit (processing dimension of less than 0.25 micrometer) in 1990s, a KrF excimer laser (248 nanometers), a shorter wavelength than an i-line (365 nanometers), was used in place of the i-line as an exposure light source. However, in production of DRAMs with an integration of 256 M and higher than 1 G which require further miniaturized process technologies (processing dimension of equal to or less than 0.2 micrometer), a light source with a further short wavelength is required, and thus a photo lithography using an ArF excimer laser (193 nanometers) has been investigated seriously since about a decade ago. At first, an ArF lithography was planned to be applied to a device starting from a 180 nanometers node device, but a KrF excimer laser lithography lived long to a weight production of a 130 nanometers node device, and thus a full-fledged application of an ArF lithography will start from a 90 nanometers node. Further, a study of a 65 nanometers node device by combining with a lens having an increased numerical aperture (NA) till 0.9 is now underway. Further shortening of wavelength of an exposure light is progressing towards the next 45 nanometers node device, and for that an F2-lithography with a 157 nanometers wavelength became a candidate. However, there are many problems with an F2-lithography; a cost-up of a scanner due to the use of a large quantities of expensive CaF2 single crystals for a projector lens, extremely poor sustainability of a soft pellicle, which leads to a change of an optical system due to introduction of a hard pellicle, a decrease in an etching resistance of a resist film, and the like. Because of these problems, it was proposed to postpone an F2-lithography and to introduce an ArF immersion lithography earlier (Proc. SPIE Vol. 4690, xxix).
In an ArF immersion lithography, a proposal is made to impregnate water between a projector lens and a wafer. A refractive index of water at 193 nanometers is 1.44, and therefore a patterning is possible even if a lens with a numerical aperture (NA) of equal to or more than 1.0 is used, and moreover, theoretically NA may be increased to nearly 1.44. In the beginning, deterioration of a resolution and a shift of a focus due to a change of refractive index associated with a change of water temperature were pointed out. However, the problems associated with the change in the refractive index have been solved by controlling the water temperature within 1/100° C. In addition, it was also confirmed that the effect of heat generation from a resist film by exposure was almost insignificant. As to the concern of water microbubbles in a pattern reversal, it was also confirmed that evolution of bubbles from a resist film by exposure was insignificant if water is fully degassed. In the early period of an immersion lithography in 1980s, a proposal was made to immerse an entire stage into water. However, a partial fill method having nozzles of water supply and drainage in which water is introduced only between a projector lens and a wafer in order to meet the movement of a high-speed scanner was adopted. By using an immersion in water, designing of a lens with NA of 1 or higher became theoretically possible. However, there appeared a problem in it that a lens of an optical system based on a conventional refractive index system becomes extraordinary large, thereby leading to distortion of a lens due to its own weight. A proposal was made to design a catadioptric optical system for a more compact lens, which accelerated to design a lens having NA of 1.0 or more. Now a possibility of a 45 nanometers node is shown by combining a lens having NA of 1.2 or more with a super resolution technology (Proc. SPIE Vol. 5040, p. 724), and furthermore a development of a lens with NA 1.35 is underway.
As a 32 nanometers node lithography technique, a lithography of a vacuum ultraviolet beam (EUV) with a wavelength of 13.5 nanometers is known. Problems of an EUV lithography are requirements for a higher laser output power, a higher sensitivity of a resist film, a higher resolution, a lower line width roughness (LWR), a non-defective MoSi laminate mask, a lower aberration of a reflective mirror, and the like, and thus there are mounting problems to be addressed.
A maximum resolution in a water-immersion lithography using a lens with NA of 1.35 is 40 to 38 nanometers, and there is no possibility to reach 32 nanometers. Accordingly, development of a material having a higher refractive index is underway to increase NA further. A limiting factor of NA in a lens is determined by a minimum refractive index among a projector lens, a liquid, and a resist film. In the case of a water immersion, a refractive index of water is the lowest as compared with a projector lens (refractive index of a synthetic quartz is 1.5) and a resist film (refractive index of a conventional methacrylate is 1.7), and thus NA of the projector lens has been determined by a refractive index of water. Recently, a highly transparent liquid having a refractive index of 1.65 is being developed. In this case, a refractive index of a projector lens made of a synthetic quartz is the lowest, and thus a material for a projector lens with a high refractive index needs to be developed. A refractive index of LUAG (Lu3Al5O12) is equal to or more than 2, and thus it is expected as the most promising material, but has problems of a large double refraction and absorption. In addition, even though a projector lens material with a refractive index of 1.8 or more is developed, NA of 1.55 is the highest with a liquid having refractive index of 1.65, thereby with it, 35 nanometers may be resolved, but 32 nanometers not. To resolve 32 nanometers, a liquid with a refractive index of 1.8 or more and a resist and a protecting film with a refractive index of 1.8 or more are necessary. The biggest problem of a material with a refractive index of 1.8 or more lies in a liquid with a high refractive index, because an absorption and a refractive index are in a trade-off relationship, and accordingly a material for it has not been found yet. In case of an alkane compound, a bridged cyclic compound is preferable than a linear compound in order to increase a refractive index, but a cyclic compound has a problem that it cannot follow a high-speed scanning of a stage of an exposure instrument because of its high viscosity. In addition, if a liquid having a refractive index of 1.8 or more is developed, the minimum refractive index lies in a resist film, and therefore, a resist film with the refractive index of 1.8 or more is also needed.
Recently, a double patterning process, in which a pattern is formed by a first exposure and development, and a second pattern is formed exactly in the space of the first pattern by a second exposure, is drawing an attention (Proc. SPIE Vol. 5754, p. 1508 (2005)). Many processes are proposed as the double patterning method. For example, there is a method in which a photo resist pattern with a line-and-space interval of 1:3 is formed by a first exposure and development, an underlying hard mask is processed by a dry etching, an another hard mask film is formed on it by exposure and development of the photo mask film to form a line pattern in a space formed by the first exposure, and then the hard mask is dry-etched to form a line-and-space pattern with a half width of the first pattern pitch. There is also another method in which a photo resist pattern with a line-and-space interval of 1:3 is formed by a first exposure and development, an underlying hard mask is processed by a dry etching, a photo resist film is coated on it, the second exposure is made on a remaining part of the hard mask, and then the hard mask is dry-etched. In both methods, hard masks are processed by two dry-etching steps.
In the former methods, the hard mask needs to be made twice. In the latter method, only one film of the hard mask is needed, but a trench pattern, in which a resolution is more difficult as compared with a line pattern, needs to be formed. In the latter method, a negative resist composition may be used for the formation of the trench pattern. With this method, a high-contrast light similar to that used to form a line by a positive pattern may be used. However, a negative resist composition has a lower dissolution contrast as compared with the positive resist composition, and thus, the negative resist composition gives a lower resolution as compared with the case in which the line is formed by the positive resist composition when the negative resist composition is used to form the same dimension of the trench pattern. In the latter method, it may be possible to apply a thermal flow method in which a wide trench pattern is formed by using a positive resist composition, and then the trench pattern is shrunk by heating a substrate, and a RELACS method in which a water-soluble film is coated on a trench pattern after development, and then the trench is shrunk by a thermal crosslink of a resist film surface. In these methods, however, there are problems of deterioration of a proximity bias and a low throughput due to further complicated processes.
In the both former and latter methods, two etchings are necessary in the substrate processing, thereby causing problems of a lower throughput as well as a deformation and a misalignment of the pattern due to the two etchings.
To perform the etching only once, there is a method in which a negative resist composition is used in the first exposure and a positive resist composition is used in the second exposure. There is another method in which a positive resist composition is used in the first exposure and a negative resist composition dissolved in a higher alcohol which has 4 or more carbon atoms and does not dissolve the positive resist composition is used in the second exposure. In these methods, the resolution is deteriorated due to the use of a negative resist composition having a low resolution.
Not to perform PEB (post exposure bake) and development between the first and the second exposure is the simplest method with a high throughput. In this case, after the first exposure, the second exposure is done on the exchanged mask having a displaced pattern, which is followed by PEB, development and dry etching. However, a photo energy of the first exposure is compensated by a photo energy of the second exposure, leading to a zero contrast, and thus a pattern is not formed. In this case, it is reported that, when an acid is generated in a nonlinear fashion by using an acid-generator which absorbs two photons or by using a contrast enhancement lithography (CEL), the energy compensation is relatively small even if an exposure is displaced by a half-pitch, and thus a pattern with a corresponding half-pitch displacement is formed even though the contrast is small (Jpn. J. App. Phys., Vol. 33 (1994), p. 6874-6877, Part 1, No. 12B, December 1994). In this case, if a mask is changed in every exposure, a throughput is remarkably deteriorated, and thus the second exposure is done after the first exposure is done with a certain collective amounts. However, in this case, a care is necessary for a dimensional change and the like caused by an acid-diffusion between the first and the second exposures.
The most critical problem in the double patterning is the overlay accuracy of the first and the second patterns. A magnitude of the position displacement corresponds to variation of the line dimension. Thus, for example, to form the 32-nanometers line with 10% accuracy, the overlay accuracy within 3.2 nanometers is necessary. Because the overlay accuracy of the present scanner is about 8 nanometers, a substantial improvement in accuracy is necessary.
Technologies to form a narrow space pattern and a small hole pattern include, not only a double patterning method, but also a afore-mentioned method using a negative resist, a thermal flow method, and a RELACS method. However, there have been problems in these methods; the resolution is low with the negative resist, and the thermal flow method and the RELACS method tend to easily vary in its dimension at a time of thermal shrink.
A method for reversing a positive pattern to a negative pattern has been known for long. For example, in Japanese Patent Laid-Open (kokai) No. H2-154266 and Japanese Patent Laid-Open (kokai) No. H6-27654, a naphthoquinone resist capable of doing a pattern reversal is proposed. A method to leave a FIB-cured part behind by an all-out exposure followed thereafter (Japanese Patent Laid-Open (kokai) No. S64-7525), and a method in which an indene carboxylic acid formed by exposing a light on a naphthoquinone diazide sensitizer is made alkali-insoluble by converting it to indene by a thermal treatment in the presence of a base, and then a positive-negative reversal is executed by an all-out exposure (Japanese Patent Laid-Open (kokai) No. H1-191423 and Japanese Patent Laid-Open (kokai) No. H1-92741), are proposed.
In the positive-negative reversal method by changing a developer, a method to obtain a negative pattern by developing hydroxystyrene partially protected by t-BOC (tert-butoxycarbonyl group) in an organic solvent or by a supercritical carbon dioxide are proposed.
A positive-negative reversal method using a silicon-containing material, in which a space part of a positive resist pattern is covered by a silicon-containing film, then the positive-negative reversal is made by removing a positive pattern part by using an oxygen gas etching to obtain a film pattern containing a silicon, thereby forming a fine hole pattern, is proposed (Japanese Patent Laid-Open (kokai) No. 2001-92154 and Japanese Patent Laid-Open (kokai) No. 2005-43420).
Miniaturization of a hole pattern is more difficult than a line pattern. To form fine holes by a conventional method, when a positive resist film is combined with a hole pattern mask and the pattern is formed by an under exposure, an exposure margin is extremely narrow. Accordingly, a method in which a large hole is formed and developed, and then shrunk the hole by a thermal flow method, a RELACS method, and the like, is proposed. However, a dimensional difference between after development and after shrink is large, and thus there is a problem of a decreased control precision with an increase of the dimensional shrink. A method in which a line pattern is formed by using a positive resist film in the X-direction by a dipole illumination, the resist pattern is cured, a resist composition is coated again on it, and then a line pattern in the Y-direction is exposed by a dipole illumination to form a hole pattern through a clearance of a latticed line pattern is proposed (Proc. SPIE Vol. 5377, p. 255 (2004)). Although a hole pattern with a large margin may be formed by combining X and Y lines by using a dipole illumination having a high contrast, etching of line patterns arranged one above the other with a high dimensional precision is difficult.
To make dimension of a hole small by this method, a wide line and a narrow space need to be formed. However, this is difficult also from a theoretical viewpoint, because a sufficient optical contrast to resolve a fine space cannot be obtained by using a positive resist.